With candy thermometer in place, cook, without stirring, until temperature reaches 235 F or soft ball stage (when a small amount of mixture dropped into very cold water forms a ball which flattens on removal from water). Remove saucepan from heat.

Without stirring, allow mixture to cool to 110 F or lukewarm, about 45 minutes (bottom of pan will feel comfortably warm).

Stir in marshmallow creme and peanut butter. With a wooden spoon, beat mixture until it holds its shape and loses some of its gloss, about 5 to 7 minutes. Turn fudge into prepared pan. Allow fudge to set, about 30 minutes.